'Now Cameron must explain, somehow, why he is still on track to achieve this meaningless, back-of-an-envelope pledge.' Photograph: Steve Parsons/PA

“It would be my tuition fees!” So David Cameron once said to the deputy prime minister during a tense exchange about pensioner benefits. Like George Osborne, Nick Clegg believes there is scope to limit the benefits given to the elderly and to target help at lower-income pensioners. But the PM won’t hear of it. The “grey vote”, he believes, would desert the Tories en masse. It would be a milestone of treachery on a par with the Lib Dems’ U-turn on university tuition fees.

Yet there are other candidates for this inglorious honour – not least the publication of the most recent set of quarterly migration statistics which means that the net migration figures available to voters on 7 May will be 54,000 higher than they were when Cameron made his now-notorious pledge: in January 2010, the Tory leader said: “We would like to see net immigration in the tens of thousands rather than the hundreds of thousands.” Even at the time, this struck me as daft, and on so many levels.

First, it was a step back from the authentic Tory modernising position that immigration was a social and economic good, to be welcomed, rather than tolerated and minimised. Second, it betrayed an ignorance of how unpredictable levels of immigration can be, given the sheer complexity of the rules and regulations governing border control, and the patchiness of their application – an odd mistake for a former Home Office special adviser to make.

Third, the extent of free movement within the (expanding) European Union makes all such promises no more meaningful than a pledge by the mayor of Witney, Cameron’s constituency, to exclude incomers from Bampton and Minster Lovell. Fourth: if the Tories did indeed preside over a jobs-led recovery as, in 2010, they hoped to do, it was inevitable that the numbers of EU migrants coming here to work would, by itself, make Cameron’s pledge redundant. He would be the victim of his own success.

Theresa May has laboured to close loopholes, to shut down scams, to crack down on the horrors of human trafficking. But the realities of modern statehood – and modern economics – do not submit to such legislative nibbles. The writers of The Thick of It were on to something when they proposed hiving off immigration policy to a non-political agency.

As it is, the issue remains simultaneously toxic and fissile. Lurking behind this particular embarrassment is the Tories’ fear of Ukip, a party posturing as a protector of British sovereignty but truly a reactionary force at odds with the pluralism of modern life. One of many reasons that the PM is keen to avoid television debates is that he does not want to give Nigel Farage the spotlight. To this day, he shudders when he recalls the 2010 Conservative campaign, which had many themes and therefore none. This time, aided by Lynton Crosby, he has decided to base his claim to a second term in No 10 on a binary choice: “Tory competence” versus “Labour chaos”. No distractions are permitted. London’s new runway? After the election. European negotiations? After the election. Recently urged in private to take a public, ethical stand against Ukip and show what distinguished statesmanship from saloon bar atavism, he declined: “I don’t want to give Farage a platform.”

All this restraint might be impressive if it were not delusional. Election campaigns, like life, do not respect plans or “grids”. Now Cameron must explain, somehow, why he is still on track to achieve this meaningless, back-of-an-envelope pledge. Immigration will flare up as an election issue and the Tories will again be on the defensive. It was not Cameron’s fault that Malcolm Rifkind acted as he did, with all the attendant embarrassment for the Conservatives as the alleged party of greed and venality. But this fiasco was all the PM’s own work.